1. Field of the Invention
The present invention relates generally to a sector drive assembly for a shutter of a camera and the like and, more particularly, to a sector drive assembly for driving, for example, a light-shielding device of a digital camera.
2. Description of the Related Art
In the related art, the initial position of a sector drive assembly for a camera is a position where a plurality of sectors are fully-closed. In order to detect the initial position of the sectors, a home switch comprised of a spring and a spring bearing has been proposed, as disclosed, for example, in Japanese Patent Document JP-A-2002-196392. The spring of the home switch is supported by a shaft secured to a base plate, the spring being fixed at one end and free at the other end. The spring bearing is fitted onto the shaft and secured to the base plate at a position where the free end of the spring can resiliently come into and out of contact therewith at a predetermined resilient force. A state in which the free end is out of contact with the spring bearing is a state in which the home switch is OFF, and is set as the initial position of a sector drive ring.
At the initial position where the home switch is OFF, when a rotor shaft and a pinion gear are rotated by an actuator, in association with the rotation, the pinion of the gear rotates to rotate the sector drive ring. The sectors start to open by the rotation of the sector drive ring. At the same time, a drive pin moves clockwise, and a pressing force of a switch driving lever toward an arm is released. Then, the switch driving lever becomes free and releases the pressing force exerted on the free end of the spring. Therefore, the free end of the spring comes into resilient contact with the spring bearing at a predetermined resilient force determined by the resiliency of the spring to turn the home switch ON. When the home switch is turned from the OFF state to the ON state, the releasing action of the sectors is detected. Then, when a required exposure time has elapsed, the actuator is rotated in the reverse direction to bring the sector drive ring back to the initial position, and the home switch is turned from the ON state to the OFF state, thereby detecting that the sectors have returned to the initial position.
In the case of a focal-plane shutter, a switch as described above may be provided at an operating limit of each sector drive lever of the front curtain and the rear curtain, respectively, for detecting the termination of travel of the sector drive levers.
In the proposed home switch described above, the initiation of movement or the return to the initial position of the sectors are detected by a single switch. That is, after the initiation of movement of the sectors, open control is performed until the sectors return to the initial state. Since it is open-controlled, drive control during that period is determined with a significant margin for the actual operation. Since environmental conditions such as ambient temperature changes, variations in battery voltage, age deterioration (durability) of components, and the like, must be taken into consideration, an electromagnetic actuator is supplied with redundant power for a predetermined period of time even after the actual shutter operation has been terminated. The same applies in the focal-plane shutter. The termination of travel of each sector drive lever is detected respectively by each of the individual switches, while performing open control, in which only the termination of travel of the sectors or the lever is detected. A problem arises based on the use of plural switches in that additional space for providing plural switches is not available in the shutter, which must be downsized.